The present invention relates to fluorescent lamps and, more particularly, to a novel two-phosphor-layer fluorescent lamp utilizing a second layer of a phosphor having a high quantum efficiency for increased brightness and minimized brightness depreciation.
A typical fluorescent lamp comprises an envelope of material transparent to visible light; a quantity of mercury vapor contained in the envelope and caused, via electrical discharge and the like mechanisms, to emit radiation at wavelengths predominantly in the ultraviolet region; and a layer of phosphor deposited upon the interior surface of the envelope for converting the ultraviolet radiation to radiation in the visible portion of the electromagnetic spectrum. Typical phosphors, such as the calcium halophosphates utilized to achieve visible light having a standard "cool white" color point, convert ultraviolet radiation having wavelengths in the region about 254 nanometer (nm.) to visible radiation; other mercury-emitted radiation at 185 nm. is not converted to visible light by the phosphor, and also has been observed to depreciate the visible light conversion output of the phosphor by as much as 10% in the first few minutes of operation. A fluorescent lamp capable of utilizing the 185 nm. mercury vapor radiation, while substantially reducing the depreciation in halophosphate phosphor efficacy (presently experienced), is therefore extremely desirable.